The present invention is directed generally to asynchronous motors and, more particularly, to cylindrical and linear asynchronous motors having simple rotor structures.
Conventional induction motors typically include a stator that is electrically connected to drive circuitry, which supplies voltage and current to the stator. Drive circuitry may have variable current, voltage and frequency capabilities. The voltage supplied to the stator creates running magnetic fields that a rotor absorbs and that create currents in the rotor. The currents in the rotor are phase shifted relative to the running magnetic field. The interaction of the induced currents and the running magnetic field creates a force that causes the rotor to move with respect to the stator. In cylindrical motors, this motion is rotational in the azimuth direction. Conversely, in linear motors, this motion is linear with respect to the stator.
Conventionally, a rotor of an asynchronous motor accounts for approximately 35-65 percent of the entire mass of the motor mass. The inertia associated with a spinning rotor is quite large and a large amount of force is needed to start the rotor spinning or to change the speed or direction of the rotor once it is spinning. For example, a variable speed asynchronous motor may consume 4-5 times more current when starting, braking and changing directions, than it consumes during constant speed steady-state operation. This current consumption requires drive circuitry that can supply increased current during starting or direction changing. Furthermore, the startup torque or low speed torque of conventional asynchronous motors is low.
Variable frequency drives may be used for starting a rotor in motion without excessive current consumption. For example, when starting a rotor in motion, the current and voltage supplied to the stator may be reduced in frequency. The reduction in frequency reduces the slip (the ratio of the difference between the speed of the rotating stator field and the speed of the rotor to the speed of the rotating stator field) of the motor, thereby allowing a lower start current to be used. However, a reduction in the frequency of the current and voltage fed to the stator reduces the startup torque available to the rotor. Accordingly, a rotor having a load that is to be turned may be unable to start using the reduced frequency technique because there is inadequate torque to turn the loaded rotor. Variable frequency drives may also be used to control the rotational speed of a rotor in an asynchronous motor. In an asynchronous motor, the lower the frequency of the voltage supplied to a stator, the slower the rotor of that motor will rotate.
One conventional asynchronous motor is the class B squirrel cage induction motor, which has a rotor containing discrete copper or aluminum conductors having their extremities connected by metal rings, wherein the conductors and the rings are embedded inside a massive, laminated ferromagnetic rotor core. A stator field induces currents onto the discrete conductors and those induced currents interact with the stator field to cause the rotor to rotate. The squirrel cage motor has a peak efficiency of approximately 98 percent at roughly 70 percent of synchronous speed. However, when a squirrel cage motor is operated at more or less than roughly 70 percent of synchronous speed, efficiency decreases. As a result, a squirrel cage motor is very inefficient in starting and stopping situations when slip is high. Furthermore, a squirrel cage motor has a torque characteristic having a peak value at approximately 70 percent of synchronous speed. Accordingly, a squirrel cage motor has very low torque in starting and in direction changing situations when the motor is operated at relatively low percentage of synchronous speed.
To alleviate some of the starting, direction changing, speed and torque problems mentioned above, some applications using conventional asynchronous motors employ a mechanical transmission between a rotor and a load. For example, a washing machine may use a mechanical transmission to couple a conventional asynchronous motor to a clothes basket that is rotated at various speeds throughout a washing cycle. The mechanical transmission compensates for the low startup torque of the motor. Additionally, the mechanical transmission enables a washing cycle having fast direction changes and agitation, which are high torque operations, to be carried out using a conventional asynchronous motor. However, the mechanical transmission increases the costs associated with using a conventional asynchronous motor.
In a first embodiment, the present invention is directed to a device for converting electrical energy into mechanical work. The device includes a rotor body having a radial length and a circumference, a ferromagnetic layer having a radial thickness, the ferromagnetic layer being disposed on the circumference of the rotor body, wherein the radial thickness of the ferromagnetic layer is less than the radial length of the rotor body and a conductive layer disposed on the ferromagnetic layer. The device further includes a stator disposed substantially along the circumference of the rotor body, the ferromagnetic layer and the conductive layer, wherein the stator is adapted to carry an alternating current signal.
In a second embodiment, the present invention is directed to a washing machine including a tank adapted to hold water, a basket disposed within the tank and adapted to hold clothes that are to be washed, the basket having a radial length and an outer circumference, a ferromagnetic layer having a radial thickness, the ferromagnetic layer being disposed on the outer circumference of the basket, wherein the radial thickness of the ferromagnetic layer is less than the radial length of the basket, a conductive layer having a radial thickness, the conductive layer being disposed on the ferromagnetic layer and a stator adjacent the tank and substantially encircling the outer circumference of the rotor body, wherein the stator is adapted to carry an alternating current signal.
In a third embodiment, the present invention is directed to a rotor for use with a stator adapted to carry an alternating current signal. The rotor includes a rotor body having a thickness, a ferromagnetic layer having a thickness, the ferromagnetic layer being disposed on the rotor body, wherein the ferromagnetic layer has a thickness equal to or greater than:       δ    Fe    =            184      ⁢      b              μ      *      
wherein:
xcex4Fe is the thickness of the ferromagnetic layer;
xcexc* is a relative magnetic permeability of the ferromagnetic layer; and
b is a radial thickness of the stator and a conductive layer being disposed on the ferromagnetic layer.
In a fourth embodiment, the present invention is directed to a device for converting electrical energy into mechanical work. The device includes a rotor body, a ferromagnetic layer disposed on the rotor body, a conductive layer disposed on the ferromagnetic layer and a stator disposed substantially along the rotor body, the ferromagnetic layer and the conductive layer. The stator has a plurality of teeth and a plurality of slots between the plurality of teeth, each tooth having a width and each slot having a width so that the ratio of the slot width to the sum of the tooth width and the slot width is about 0.7 to about 0.75.
The above and other aspects and advantages of the present invention will become apparent from the following detailed description of the present invention taken in conjunction with the drawings.